From fe9ab936b6aa646ef87c636ec8803d42f8b7bb1d Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Fri, 22 May 2026 17:48:31 +0200
Subject: [PATCH] fix: allow `$derived(await ...)` in disconnected effect roots
 (#18273)

If `$effect.root` is created outside the component tree, the boundary
property is null

Fixes #18188
---
 .changeset/fine-corners-lay.md                        |  5 +++++
 .../svelte/src/internal/client/reactivity/deriveds.js |  5 ++++-
 .../async-async-disconnected-effect-root/_config.js   |  9 +++++++++
 .../async-async-disconnected-effect-root/main.svelte  | 11 +++++++++++
 4 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fine-corners-lay.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/main.svelte

diff --git a/.changeset/fine-corners-lay.md b/.changeset/fine-corners-lay.md
new file mode 100644
index 0000000000..badbc6f81f
--- /dev/null
+++ b/.changeset/fine-corners-lay.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow `$derived(await ...)` in disconnected effect roots
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 8d240bca31..7ea6de6306 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -187,7 +187,10 @@ export function async_derived(fn, label, location) {
 				var decrement_pending = increment_pending();
 			}
 
-			if (/** @type {Boundary} */ (parent.b).is_rendered()) {
+			if (
+				// boundary can be null if the async derived is inside an $effect.root not connected to the component render tree
+				parent.b?.is_rendered()
+			) {
 				batch.async_deriveds.get(effect)?.reject(OBSOLETE);
 			} else {
 				// While the boundary is still showing pending, a new run supersedes all older in-flight runs
diff --git a/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/_config.js b/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/_config.js
new file mode 100644
index 0000000000..ecf048abc3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/_config.js
@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	// Test that an async derived inside an $effect.root not connected to the component tree still works
+	async test({ assert, logs }) {
+		await new Promise((resolve) => setTimeout(resolve, 10));
+		assert.deepEqual(logs, [1]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/main.svelte
new file mode 100644
index 0000000000..37c8919023
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-async-disconnected-effect-root/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	setTimeout(() => {
+		$effect.root(() => {
+			async function fn() {
+				const value = $derived(await 1);
+				return { get value() { return value } };
+			}
+			fn().then(r => console.log(r.value));
+		})
+	})
+</script>